CONSTRUCTION OF SYNCHRONOUS MACHINES

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UNIT IV

SYNCHRONOUS AND SPECIAL MACHINES

Construction of synchronous machines-types – Induced emf – Voltage regulation; emf and

mmf methods – Brushless alternators – Reluctance motor – Hysteresis motor – Stepper motor.

Principle of Operation

CONSTRUCTION OF SYNCHRONOUS MACHINES:

two types of alternators

(a) stationary armature - rotating field

(b) stationary field – rotating armature

advantages of stationary armature - rotating field:

i) the hv ac winding and its insulation not subjected to centrifugal forces.

ii) easier to collect large currents from a stationary member.

iii) the lv dc excitation easily supplied through slip rings and brushes to the

rotor field winding.

stationary armature - rotating field

stator:

laminated core with slots to hold the armature conductors.

rotor:

i) salient pole type –

projecting poles dove tailed on the shaft - used in low speed alternators driven by water turbines

or ic engines

ii) non salient pole type

smooth cylindrical rotor - slots cut to house the field winding - used in high speed alternators

driven by steam turbines - smaller diameter and larger axial length compared to salient pole type

machines, of the same rating.

induced emf:



Eph 4.44Kc Kd f Tph

short pitching and distribution of the winding:

time harmonics of induced voltage reduced

the waveform made more sinusoidal.

short pitching also reduces the length of the coil end connections

armature reaction

effect of the armature flux on the main flux.

three phase current in a three - phase winding - a rotating magnetic field produced (mmf = 1.5 im

tph).

upf - cross magnetizing.

lag pf - demagnetizing.

lead pf - magnetizing.

armature leakage reactance(xl)

three major components -slot leakage

reactance, end winding leakage reactance

and tooth tip leakage reactance.

synchronous reactance/phase

xs = xl + xar, where xar is the fictitious

armature reaction reactance.

synchronous impedance/phase

zs = (ra + jxs ).

voltage regulation of alternators

E ph Vph I a Z s

E ph



V

ph

2

V ph 100

E ph

V ph

ph cos Ia Ra

2

V

sin Ia X s

Methods Of Predetermination Of Regulation

• Synchronous impedance method (EMF

method)

• Magneto Motive Force method (MMF

method)

• Zero Power Factor method (ZPF method)

• American Standards Association method

(ASA method)

Synchronous Impedance Method (EMF Method)

• OC and SC tests conducted.

• Zs is calculated.

• Ra measured and Xs obtained.

• For a given armature current and power

factor, Eph determined - regulation is



calculated.

Magneto Motive Force Method (MMF Method)

• OC & SC tests conducted.

• field currents If1 (field current required to produce a voltage of(Vph + IaphRacos Φ) on

OC) and If2 (field current required to produce the given armature current on SC) are added

at an angle of (90± Φ).

• For this total field current, Eph found from OCC and regulation calculated.

Zero Power Factor Method (ZPF Method)

OC test and ZPF test is conducted – characteristics are drawn . This is Potier

triangle method

From this triangle the potier reactance (leakage reactance of the alternator), XLph

is obtained.

• The terminal voltage and the leakage reactance drop added vectorially -

load induced EMF

found.

• For this load induced emf, the corresponding field current If1 obtained

from OCC.

• The field current If2 required to balance armature reaction obtained from

potier triangle.

• If1 and If2 are added at an angle of (90± Φ). For this total field current,

Eph found from

OCC - regulation calculated.

Synchronizing Power and Torque

• Power developed by an alternator



Where

θ is the internal angle of the machine and

δ is the power angle.

Synchronizing power

On no load .

Synchronizing torque = PSY / ( 2πNs/60).

Stepper motor

Fundamentals of operation

Stepper motors operate differently from DC brush motors, which rotate when voltage is applied to

their terminals. Stepper motors, on the other hand, effectively have multiple "toothed"

electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are

energized by an external control circuit, such as a microcontroller. To make the motor shaft turn,

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first one electromagnet is given power, which makes the gear's teeth magnetically attracted to the

electromagnet's teeth. When the gear's teeth are thus aligned to the first electromagnet, they are

slightly offset from the next electromagnet. So when the next electromagnet is turned on and the

first is turned off, the gear rotates slightly to align with the next one, and from there the process is

repeated. Each of those slight rotations is called a "step", with an integer number of steps making a

full rotation. In that way, the motor can be turned by a precise angle.

Stepper motor characteristics

1. Stepper motors are constant power devices.

2. As motor speed increases, torque decreases. (most motors exhibit maximum torque when

stationary, however the torque of a motor when stationary 'holding torque' defines the

ability of the motor to maintain a desired position while under external load).

3. The torque curve may be extended by using current limiting drivers and

increasing the

driving voltage (sometimes referred to as a 'chopper' circuit, there are several off the shelf

driver chips capable of doing this in a simple manner).

4. Steppers exhibit more vibration than other motor types, as the discrete step

tends to snap

the rotor from one position to another (called a detent). The vibration

makes stepper motors

noisier than DC motors.

5. This vibration can become very bad at some speeds and can cause the

motor to lose torque

or lose direction. This is because the rotor is being held in a magnetic field which behaves

like a spring. On each step the rotor overshoots and bounces back and forth, "ringing" at its

resonant frequency. If the stepping frequency matches the resonant frequency then the

ringing increases and the motor comes out of synchronism, resulting in positional error or a

change in direction. At worst there is a total loss of control and holding torque so the motor

is easily overcome by the load and spins almost freely.

6. The effect can be mitigated by accelerating quickly through the problem speeds range,

physically damping (frictional damping) the system, or using a micro-stepping driver.

7. Motors with a greater number of phases also exhibit smoother operation than those with

fewer phases (this can also be achieved through the use of a micro stepping drive)



Open-loop versus closed-loop commutation

Steppers are generally commutated open loop, i.e. the driver has no feedback on where the rotor

actually is. Stepper motor systems must thus generally be over engineered, especially if t he load

inertia is high, or there is widely varying load, so that there is no possibility that the motor will lose

steps. This has often caused the system designer to consider the trade-offs between a closely sized

but expensive servomechanism system and an oversized but relatively cheap stepper.

A new development in stepper control is to incorporate a rotor position feedback (e.g. an encoder

or resolver), so that the commutation can be made optimal for torque generation according to

actual rotor position. This turns the stepper motor into a high pole count brushless servo motor,

with exceptional low speed torque and position resolution. An advance on this technique is to

normally run the motor in open loop mode, and only enter closed loop mode if the rotor position

error becomes too large — this will allow the system to avoid hunting or oscillating, a common

servo problem.

http://en.wikipedia.org/wiki/Servomechanism

http://en.wikipedia.org/wiki/Encoder

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Types

There are three main types of stepper motors:[1]

1. Permanent Magnet Stepper (can be subdivided in to 'tin-can' and 'hybrid', tin-can being a

cheaper product, and hybrid with higher quality bearings, smaller step angle, higher power

density)

2. Hybrid Synchronous Stepper

3. Variable Reluctance Stepper

4. Lavet type stepping motor

Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the attraction

or repulsion between the rotor PM and the stator electromagnets. Variable reluctance (VR) motors

have a plain iron rotor and operate based on the principle that minimum reluctance occurs with

minimum gap, hence the rotor points are attracted toward the stator magnet poles. Hybrid stepper

motors are named because they use a combination of PM and VR techniques to achieve maximum

power in a small package size.

Two-phase stepper motors

There are two basic winding arrangements for the electromagnetic coils in a two phase stepper

motor: bipolar and unipolar.

Unipolar motors

A unipolar stepper motor has two windings per phase, one for each direction of magnetic field.

Since in this arrangement a magnetic pole can be reversed without switching the direction of

current, the commutation circuit can be made very simple (eg. a single transistor) for each

winding. Typically, given a phase, one end of each winding is made common: giving three leads

per phase and six leads for a typical two phase motor. Often, these two phase commons are

internally joined, so the motor has only five leads.

A microcontroller or stepper motor controller can be used to activate the drive transistors in the

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http://en.wikipedia.org/wiki/Lavet_type_stepping_motor




right order, and this ease of operation makes unipolar motors popular with hobbyists; they are

probably the cheapest way to get precise angular movements.

Unipolar stepper motor coils

(For the experimenter, one way to distinguish common wire from a coil-end wire is by measuring

the resistance. Resistance between common wire and coil-end wire is always half of what it is



between coil-end and coil-end wires. This is because there is twice the length of coil between the

ends and only half from center (common wire) to the end.) A quick way to determine if the stepper

motor is working is to short circuit every two pairs and try turning the shaft, whenever a higher

than normal resistance is felt, it indicates that the circuit to the particular winding

is closed and that

the phase is working.

Bipolar motor

Bipolar motors have a single winding per phase. The current in a winding needs to be reversed in

order to reverse a magnetic pole, so the driving circuit must be more complicated, typically with an

H-bridge arrangement (however there are several off the shelf driver chips available to make this a

simple affair). There are two leads per phase, none are common.

Static friction effects using an H-bridge have been observed with certain drive

topologies

Because windings are better utilized, they are more powerful than a unipolar motor of the same

weight. This is due to the physical space occupied by the windings. A unipolar motor has twice the

amount of wire in the same space, but only half used at any point in time, hence is 50% efficient

(or approximately 70% of the torque output available). Though bipolar is more

complicated to

drive, the abundance of driver chip means this is much less difficult to achieve.

An 8-lead stepper is wound like a unipolar stepper, but the leads are not joined to common

internally to the motor. This kind of motor can be wired in several configurations:

Unipolar.

Bipolar with series windings. This gives higher inductance but lower

current per winding.

Bipolar with parallel windings. This requires higher current but can perform better as the

winding inductance is reduced.

Bipolar with a single winding per phase. This method will run the motor on only half the

available windings, which will reduce the available low speed torque but require less

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current.

Higher-phase count stepper motors

Multi-phase stepper motors with many phases tend to have much lower levels of vibration,

although the cost of manufacture is higher. These motors tend to be called 'hybrid' and have more

expensive machined parts, but also higher quality bearings. Though they are more expensive, they

do have a higher power density and with the appropriate drive electronics are actually better suited

to the application[citation needed]

, however price is always an important factor. Computer printers

may use hybrid designs.

Stepper motor drive circuits

Stepper motor performance is strongly dependent on the drive circuit. Torque curves may be

extended to greater speeds if the stator poles can be reversed more quickly, the limiting factor

being the winding inductance. To overcome the inductance and switch the windings quickly, one

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must increase the drive voltage. This leads further to the necessity of limiting the current that

these high voltages may otherwise induce.

L/R drive circuits

L/R drive circuits are also referred to as constant voltage drives because a constant positive or

negative voltage is applied to each winding to set the step positions. However, it is winding

current, not voltage that applies torque to the stepper motor shaft. The current I in each

winding is related to the applied voltage V by the winding inductance L and the winding

resistance R. The resistance R determines the maximum current according to Ohm's law

I=V/R. The inductance L determines the maximum rate of change of the current in the winding

according to the formula for an Inductor dI/dt = V/L. Thus when controlled by an L/R drive,

the maximum speed of a stepper motor is limited by its inductance since at some speed, the

voltage U will be changing faster than the current I can keep up. In simple terms the rate of

change of current is L X R (e.g. a 10mH inductance with 2 ohms resistance will take 20 ms to

reach approx 2/3rds of maximum torque or around 0.1 sec to reach 99% of max torque). To

obtain high torque at high speeds requires a large drive voltage with a low resistance and low

inductance. With an L/R drive it is possible to co ntrol a low voltage resistive motor with a

higher voltage drive simply by adding an external resistor in series with each winding. This

will waste power in the resistors, and generate heat. It is therefore considered a low

performing option, albeit simple and cheap.

Chopper drive circuits

Chopper drive circuits are also referred to as constant current drives because they generate a

somewhat constant current in each winding rather than applying a constant voltage. On each

new step, a very high voltage is applied to the winding initially. This causes the current in the

winding to rise quickly since dI/dt = V/L where V is very large. The current in each winding is

monitored by the controller, usually by measuring the voltage across a small sense resistor in

series with each

winding. When the current exceeds a specified current limit, the voltage is

turned off or "chopped",

typically using power transistors. When the winding current drops below the specified limit,

the voltage is turned on again. In this way, the current is held relatively constant for a

http://en.wikipedia.org/wiki/Ohm%27s_law

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particular step position. This requires additional electronics to sense winding currents, and

control the switching, but it allows stepper motors to be driven with higher torque at higher

speeds than L/R drives. Integrated electronics for this purpose are widely available.

Phase current waveforms

A stepper motor is a polyphase AC synchronous motor , and it is ideally driven by

sinusoidal current. A full step waveform is a gross approximation of a sinusoid, and is the

reason why the motor exhibits so much vibration. Various drive techniques have been

developed to better approximate a sinusoidal drive waveform: these are half stepping and

microstepping.

http://en.wikipedia.org/wiki/Electric_motor#Three-phase_AC_synchronous_motors



Different drive modes showing coil current on a 4-phase unipolar stepper motor

Full step drive (two phases on)

This is the usual method for full step driving the motor. Both phases are always on. The motor will

have full rated torque.

Wave drive

In this drive method only a single phase is activated at a time. It has the same number of steps as

the full step drive, but the motor will have significantly less than rated torque. It is rarely used.

Half stepping

When half stepping, the drive alternates between two phases on and a single phase on. This

increases the angular resolution, but the motor also has less torque (approx 70%) at the half step

position (where only a single phase is on). This may be mitigated by increasing the current in the

active winding to compensate. The advantage of half stepping is that the drive electronics need not

change to support it.



Microstepping

What is commonly referred to as microstepping is actually "sine cosine microstepping" in which

the winding current approximates a sinusoidal AC waveform. Sine cosine microstepping is the

most common form, but other waveforms are used [1]. Regardless of the waveform used, as the

microsteps become smaller, motor operation becomes more smooth, thereby greatly reducing

resonance in any parts the motor may be connected to, as well as the motor itself. Resolution will

be limited by the mechanical stiction, backlash, and other sources of error between the motor and

the end device. Gear reducers may be used to increase resolution of positioning.

http://www.zaber.com/wiki/Tutorials/Microstepping

http://en.wikipedia.org/wiki/Stiction



Step size repeatability is an important step motor feature and a fundamental reason for their use in

positioning.

Example: many modern hybrid step motors are rated such that the travel of every full step

(example 1.8 Degrees per full step or 200 full steps per revolution) will be within 3% or 5% of the

travel of every other full step; as long as the motor is operated within its specified operating

ranges. Several manufacturers show that their motors can easily maintain the 3% or 5% equality of

step travel size as step size is reduced from full stepping down to 1/10th stepping. Then, as the

microstepping divisor number grows, step size repeatability degrades. At large step size reductions

it is possible to issue many microstep commands before any motion occurs at all and then the

motion can be a "jump" to a new position.

Theory

A step motor can be viewed as a synchronous AC motor with the number of poles (on both rotor

and stator) increased, taking care that they have no common denominator. Additionally, soft

magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles

(reluctance motor). Modern steppers are of hybrid design, having both permanent magnets and soft

iron cores.

To achieve full rated torque, the coils in a stepper motor must reach their full rated current during

each step. Winding inductance and reverse EMF generated by a moving rotor tend to resist

changes in drive current, so that as the motor speeds up, less and less time is spent at full current

— thus reducing motor torque. As speeds further increase, the current will not reach the rated

value, and eventually the motor will cease to produce torque.

Pull-in torque

This is the measure of the torque produced by a stepper motor when it is operated without an

acceleration state. At low speeds the stepper motor can synchronise itself with an applied step

frequency, and this pull-in torque must overcome friction and inertia. It is important to make sure

that the load on the motor is frictional rather than inertial as the friction reduces any unwanted

oscillations.

http://en.wikipedia.org/wiki/Current_(electricity)



Pull-out torque

The stepper motor pull-out torque is measured by accelerating the motor to the desired speed and

then increasing the torque loading until the motor stalls or misses steps. This measurement is taken

across a wide range of speeds and the results are used to generate the stepper motor's dynamic

performance curve. As noted below this curve is affected by drive voltage, drive current and

current switching techniques. A designer may include a safety factor between the rated torque and

the estimated full load torque required for the application.

Detent torque



Synchronous electric motors using permanent magnets have a remnant position holding

torque (called detent torque or cogging, and sometimes included in the specifications) when

not driven electrically. Soft iron reluctance cores do not exhibit this behavior.

Stepper motor ratings and specifications

Stepper motors nameplates typically give only the winding current and occasionally the

voltage and winding resistance. The rated voltage will produce the rated winding current at

DC: but this is mostly a meaningless rating, as all modern drivers are current limiting and the

drive voltages greatly exceed the motor rated voltage.

A stepper's low speed torque will vary directly with current. How quickly the torque falls off

at faster speeds depends on the winding inductance and the drive circuitry it is attached to,

especially the driving voltage.

Steppers should be sized according to published torque curve, which is specified by

the manufacturer at particular drive voltages or using their own drive circuitry.

Applications

Computer-controlled stepper motors are one of the most versatile forms of positioning

systems. They are typically digitally controlled as part of an open loop system, and are

simpler and more rugged than closed loop servo systems.

Industrial applications are in high speed pick and place equipment and multi-axis machine

CNC machines often directly driving lead screws or ballscrews. In the field of lasers and optics

they are frequently used in precision positioning equipment such as linear actuators, linear

stages, rotation stages, goniometers, and mirror mounts. Other uses are in packaging

machinery, and positioning of valve pilot stages for fluid control systems.

Commercially, stepper motors are used in floppy disk drives, flatbed scanners, computer

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printers, plotters, slot machines, and many more devices.

THE HYSTERESIS MOTOR

The hysteresis synchronous motor is so named because it utilizes the phenomenon of

hysteresis to produce mechanical torque. In its simplest form, the rotor of a hysteresis motor is

a smooth cylindrical tube of high hysteresis loss permanent magnet material without windings

or slots. It is placed within a slotted stator carrying distributed windings designed to produce,

as nearly as possible, a sinusoidal space distribution of flux. In single phase motors, the stator

windings usually are the permanent-split-capacitor type. The capacitor value is selected to

result in approximately balanced 2 phase conditions within the motor windings: The stator then

produces a rotating field, approximately constant in space wave form and rotating at

synchronous speed.

Instantaneous magnetic conditions in the air gap are indicated in Figure 1 for a 2 pole stator. The

axis SS' of the stator m.m.f. wave revolves at synchronous speed, because of hysteresis the

magnetization of the rotor lags behind the inducing m.m.f. wave, and therefore the axis RR' of

the rotor flux wave lags behind the axis of the stator m.m.f. wave by the hysteresis lag angle "a".

If the rotor is stationary, starting torque is produced proportional to the product of the

fundamental components of the stator m.m.f. and rotor flux and the sine of the torque angle "a".

The rotor :hen accelerates if the counter torque of the load is less than the developed torque of



http://en.wikipedia.org/wiki/Slot_machine



the motor. When the rotor is turning at less than synchronous speed, each particle of the rotor is

subjected to a repetitive hysteresis cycle at slip frequency. While the rotor is accelerating, the lag

angle "a" remains constant if the flux is constant, since the angle "a" depends merely on the

hysteresis loop of the rotor and is independent of the rate at which the loop is traversed.

The motor therefore develops constant torque right up to synchronous speed, as shown in the

ideal speed torque, Figure 2. This feature is one of the advantages of the hysteresis motor in

contrast to a reluctance motor which must snap its load into synchronism from the induction

motor torque speed characteristic. Hysteresis motors can synchronize any load they can

accelerate, regardless of the inertia. After reaching synchronism the motors continue to run at

synchronous speed and adjust their torque angle to develop torques required by the loads.

There are deviations from the ideal speed torque curves for several reasons. In a single phase

capacitor motor, a true two phase condition occurs only at one load point. It is not always

possible to obtain a true sinusoidal winding distribution tooth pulsation loss in the rotor etc.,

so that speed torque curves A and B can be obtained. Therefore there is some flexibility in

design possible to obtain curves A or B; curve A, when starting torque is not required; curve

B, when a high starting torque is required.

Reluctance motors

Reluctance motor is a type of synchronous electric motor that induces non-

permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the

phenomenon of magnetic reluctance.

http://en.wikipedia.org/wiki/Electric_motor

http://en.wikipedia.org/wiki/Ferromagnetic

http://en.wikipedia.org/wiki/Magnetic_reluctance



A reluctance motor, in its various incarnations, may be known as a:

Synchronous reluctance motor

Variable reluctance motor

Switched Reluctance Motor

Variable reluctance stepping motor

Reluctance motors can have very high power density at low-cost, making them ideal for

many applications. Disadvantages are high torque ripple when operated at low speed, and

noise caused by torque ripple. Until recently, their use has been limited by the complexity

inherent in both designing the motors and controlling them. These challenges are being

overcome by advances in

the theory, by the use of sophisticated computer design tools, and by the use of low-cost

embedded systems for motor control. These control systems are typically based on

microcontrollers using control algorithms and real-time computing to tailor drive waveforms

according to rotor position and current or voltage feedback.

Design and operating fundamentals

The stator consists of multiple salient (i.e., projecting) electromagnet poles, similar to a wound

field brushed DC motor. The rotor consists of soft magnetic material, such as laminated silicon

steel, which has multiple projections acting as salient magnetic poles through magnetic

reluctance. The number of rotor poles is typically less than the number of stator poles, which

minimizes torque ripple and prevents the poles from all aligning simultaneously—a position

which can not generate torque.

When a rotor pole is equidistant from the two adjacent stator poles, the rotor pole is said to be

in the "fully unaligned position". This is the position of maximum magnetic reluctance for the

rotor pole. In the "aligned position", two (or more) rotor poles are fully aligned with two (or

more) stator poles, (which means the rotor poles completely face the stator poles) and is a

position of minimum reluctance.

http://en.wikipedia.org/wiki/Switched_Reluctance_Motor

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http://en.wikipedia.org/wiki/Real-time_computing

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http://en.wikipedia.org/wiki/Reluctance

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When a stator pole is energized, the rotor torque is in the direction that will reduce reluctance.

Thus the nearest rotor pole is pulled from the unaligned position into alignment with the stator

field (a position of less reluctance). (This is the same effect used by a solenoid, or when picking

up ferromagnetic metal with a magnet.) In order to sustain rotation, the stator field must rotate

in advance of the rotor poles, thus constantly "pulling" the rotor along. Some motor variants

will run on 3-phase AC power (see the synchronous reluctance variant below). Most modern

designs are of the switched reluctance type, because electronic commutation gives significant

control advantages for motor starting, speed control, and smooth operation (low torque ripple).

Dual-rotor layouts provide more torque at lower price per volume or per

mass.[citation needed]

The inductance of each phase winding in the motor will vary with position, because the

reluctance also varies with position. This presents a control systems challenge.

Types of Reluctance motors

Synchronous reluctance

Synchronous reluctance motors do have an equal number of stator and rotor poles. The rotor

saliency is arranged by introducing internal flux ―barriers― i.e. holes which direct the magnetic

flux along the so called direct axis. Typical pole numbers are 4 and 6.

As the rotor is operating at synchronous speed and there are no current conducting parts in

the rotor, the rotor losses are minimal compared to those of induction motor.

Once started at synchronous speed, the SynRM motor can operate with sinusoidal voltage, but

the speed control requires an electronic frequency converter.

Applications

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http://en.wikipedia.org/wiki/Magnet

http://en.wikipedia.org/wiki/3-phase_AC

http://en.wikipedia.org/wiki/Commutator_(electric)

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SRM's are used in some washing machine designs.

Switched reluctance

If an iron rotor with poles, but without any conductors, is fitted to a multi-phase stator, a

switched reluctance motor, capable of synchronizing with the stator field results. When a

stator coil pole pair is energized, the rotor will move to the lowest magnetic reluctance path.

(Figure below) A

switched reluctance motor is also known as a variable reluctance motor. The

reluctance of the rotor

to stator flux path varies with the position of the rotor.

Reluctance is a function of rotor position in a variable reluctance motor.

Sequential switching (Figure below) of the stator phases moves the rotor from one position to

the next. The mangetic flux seeks the path of least reluctance, the magnetic analog of electric

resistance. This is an over simplified rotor and waveforms to illustrate operation.

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Variable reluctance motor, over-simplified operation.

If one end of each 3-phase winding of the switched reluctance motor is brought out via a

common lead wire, we can explain operation as if it were a stepper motor. (Figure above) The

other coil connections are successively pulled to ground, one at a time, in a wave drive pattern.

This attracts the rotor to the clockwise rotating magnetic field in 60o

increments.

Various waveforms may drive variable reluctance motors. (Figure below) Wave drive (a) is

simple, requiring only a single ended unipolar switch. That is, one which only switches in one

direction. More torque is provided by the bipolar drive (b), but requires a bipolar switch. The

power driver must pull alternately high and low. Waveforms (a & b) are applicable to the

stepper motor version of the variable reluctance motor. For smooth vibration free operation

the 6-step approximation of a sine wave (c) is desirable and easy to generate. Sine wave drive

(d) may be generated by a pulse width modulator (PWM), or drawn from the power line.





Variable reluctance motor drive waveforms: (a) unipolar wave drive, (b)

bipolar full step (c)

sinewave (d) bipolar 6-step.

Doubling the number of stator poles decreases the rotating speed and increases torque. This

might eliminate a gear reduction drive. A variable reluctance motor intended to move in

discrete steps, stop, and start is a variable reluctance stepper motor, covered in another

section. If smooth

rotation is the goal, there is an electronic driven version of the switched

reluctance motor. Variable

reluctance motors or steppers actually use rotors like those in Figure below.

Advantages

Simple construction- no brushes, commutator, or permanent magnets, no Cu or Al in

the rotor.

High efficiency and reliability compared to conventional AC or DC

motors.

High starting torque.

Cost effective compared to bushless DC motor in high volumes.

Adaptable to very high ambient temperature.




Low cost accurate speed control possible if volume is high enough.

Disadvantages

Current versus torque is highly nonlinear

Phase switching must be precise to minimize ripple torque

Phase current must be controlled to minimize ripple torque

Acoustic and electrical noise

Not applicable to low volumes due to complex control issues

Documents

CONSTRUCTION OF SYNCHRONOUS MACHINES